In accordance With clean air regulations it is necessary to assure during the operation of a furnace and also during the entire start-up phase that the maximally prescribed emission values for pollutants generated by combustion are not exceeded. For safety reasons the regulations furthermore require that the boiler be flushed with fresh air before every operation. Up to now this flushing is performed by a natural gas exchange, but mostly by the directed use of a blower. Accordingly, at the beginning there is a surplus of fresh air in the combustion air after each new operation in a furnace system operated by means of exhaust gas recirculation. This is the result of the fresh air in the combustion chamber being at first aspirated, instead of the flue gas which has not yet formed there. To place the burner into operation, the ignition of the mixture present at the start is absolutely required. However since, as described above, the mixture at first only consists of fresh air and fuel, i.e. the air regularly aspirated from the outside and a portion of air recirculated from the furnace and the constant amount of fuel, ignition already poses a problem because of the excess of fresh air. Thus, in order to achieve ignition at all, it is necessary to keep the rate of recirculation small in general. But the result of this is that following ignition, at a time when the fresh air in the furnace is being slowly replaced by flue gases, the combustion air, on account of the recirculation rate which is necessarily held low, does not contain sufficient amounts of flue gases per se, so as not to make it impossible to ignite the mixture during the start-up phase. However, a low flue gas recirculation never achieves the minimally possible pollutant emission values, so that the operation of furnaces with a mixture composition of this kind per se can hardly fulfill the legal requirements. Although it would easily be possible to increase the recirculation rate, this would mean that safe ignition of the combustion air/fuel mixture with an initial surplus of fresh air could no longer be assured or even made impossible. Accordingly, looking at it from this aspect, the only remaining possibility would be to operate with a reduced recirculation rate which, in turn, makes keeping the legal clean air regulations impossible.